Cardiac output is the measure of average blood flow through the heart. It is the product of the heart rate times the stroke volume. For a healthy adult in rest, a normal cardiac output is about 5 liters per minute. A healthy human body attempts to maintain a normal cardiac output by increasing or decreasing the heart rate or stroke volume. If the body cannot adequately compensate heart rate or stroke volume, adverse physiological conditions occur.
The measurement of cardiac output is an important part of health care. One method of measuring cardiac output is based on the principle of thermodilution. A catheter is inserted into a peripheral vein and the catheter tip is threaded into the pulmonary artery. Cold saline is injected through the catheter into the blood approximately three inches from the catheter tip. As the blood carries the saline past the catheter tip, a thermistor detects the decrease in temperature of the blood/saline mix. The measured temperature dilution is used to estimate cardiac output.
Another method of measuring cardiac output is based on the arterial blood pressure waveform. An advantage of this method is that the waveform can be obtained from non-invasive or minimally invasive devices.
The principle of obtaining cardiac output from blood pressure is analogous to the principle of electrical circuit analysis. Cardiac output can be derived from blood pressure if cardiovascular impedance is known, in a manner analogous to deriving electrical current from voltage if circuit impedance is known.
Various approaches have been used to provide a model for obtaining cardiac output from blood pressure. A well-known electrical analog model is known as the Windkessal model, developed in the 1930's. Linear and non-linear variations of this model have been developed.
More recently developed blood pressure models depart from analog models. For example, one approach uses pattern recognition to obtain a continuous cardiac output measurement from arterial pressure waveforms. U.S. Pat. No. 5,390,679, to Martin, entitled "Continuous Cardiac Output derived from the Arterial Pressure Waveform Using Pattern Recognition", describes this approach.